We study a model Hamiltonian for superconductivity in underdamped Josephson-junction arrays in the presence of an offset voltage between the array and the substrate. We develop an approximate zero-temperature (T=0) phase diagram as a function of Josephson coupling, charging energy, and offset voltage, using a simple Hartree-type mean-field approximation. With diagonal charging energy, the calculated phase diagram is periodic in offset voltage, in agreement with previous results. At a special value of this voltage such that states with n and n+1 Cooper pairs per grain are degenerate, only an infinitesimal Josephson coupling is needed to establish long-range phase coherence in this approximation. With both diagonal and nearest-neighbor charging energies, the T=0 phase diagram has two types of insulating lobes with different kinds of charge order, and two types of superconducting regions. One of these is a ‘‘supersolid’’ in which long-range phase coherence coexists with a frozen charge-density wave. We briefly discuss connections to previous calculations, and possible relevance to experiments.

We calculate the single-particle excitation spectrum of holes in the Emery model thereby extending and improving previous calculations. The system is considered at half filling (${\mathit{n}}_{\mathit{h}}$=1, one hole per ${\mathrm{CuO}}_2$ unit) and for hole doping, where the on-site hole-hole repulsions are kept finite. A paramagnetic form of the ground state is used. For the determination of the retarded Green’s functions of copper and oxygen holes, the projection technique is applied solving the resulting equations of motions self-consistently. At half filling, the excitation spectrum exhibits a charge-transfer gap bounded by Zhang-Rice singlet states and the upper Hubbard band. Upon hole doping the flat singlet band crosses the Fermi level giving rise to a large Fermi surface at a hole concentration of ${\mathit{n}}_{\mathit{h}}$=1.25. Moreover, spectral weight is shifted from the upper Hubbard band to the states near the Fermi energy. The calculated spectral densities, the singlet dispersion for the doped system, and the transfer of spectral weight are in good quantitative agreement with exact diagonalization results for 2×2 ${\mathrm{CuO}}_2$ cluster.

We observe a zero-field current step (ZFS) in several square Josephson tunnel junctions of intermediate size (i.e., size ≃${\lambda }_{\mathit{j}}$). This step clearly shows soliton-type motion along the two diagonals of the junction as predicted by Turlot et al. [Phys. Rev. B 42, 8418 (1990)], called isoperimetric pattern dynamics (IPD). In addition to the information given by the current-voltage characteristic, the dynamical behavior is investigated using low-temperature scanning electron microscopy (LTSEM). In parallel, extensive numerical simulations are performed to simulate the effect of the LTSEM ‘‘hot spot’’ on the IPD dynamics governed by a two-dimensional perturbed sine-Gordon equation. The comparison between the experimental data obtained by the LTSEM technique and numerical simulations clearly shows the validity of the IPD model in order to explain the observed ZFS.

Superconductivity is reported for a Zr-Nb-S κ-phase material, a compound with highly anisotropic chemical bonding. It is composed of transition-metal icosahedral clusters that are face sharing along the c axis and give metal-metal bonding in that direction. In the a-b plane, the icosahedral clusters are joined by covalent sulfur bonds. The fundamental thermodynamic properties of these superconducting ${\mathrm{Zr}}_9$${\mathrm{Nb}}_4$${\mathrm{S}}_4$ compounds have been studied in order to determine whether the anisotropic chemical bonding of this structure would lead to unusual superconducting properties. The thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$ roughly follows the BCS prediction with a transition temperature of 10.3 K, a value of the critical field at T=0 of ${\mathrm{\mu }}_0$${\mathit{H}}_0$=250 mT, and the Ginzburg-Landau parameter of about κ=30. The thermodynamic properties are very similar to transition-metal superconductors like vanadium.

Measurements of the pyroelectric effect in single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are reported as a function of doping, external electric field, and temperature. The pyroelectric response of the material is found to be accurately described theoretically. The sign pyroelectric voltage ${\mathbf{V}}_{\mathit{p}}$ can be switched by an external field. Although poling was possible to some extent, a permanent switching of the direction of polarization could not be achieved at room temperature. A sign reversal of ${\mathbf{V}}_{\mathit{p}}$ is observed as a result of thermal cycling, presumably due to activated domain rearrangement. The magnitude of the spontaneous polarization ${\mathbf{P}}_{\mathit{s}}$ is found to increase with doping in such a way that the product ρ${\mathbf{P}}_{\mathit{s}}$ remains approximately constant, where ρ is the resistivity. The origin of the polarization and the role of possible artifacts are discussed.

Two unrelated problems can be reduced to a model of a Bose gas interacting with a gauge field: (i) the effect of thermal fluctuations on a system of vortices in bulk superconductors in fields ${\mathit{H}}_{\mathit{c}1}$≪H≪${\mathit{H}}_{\mathit{c}2}$, and (ii) charged, spinless excitations in two-dimensional (2D) strongly correlated electron systems. Both problems are important for the theory of high-temperature superconductors. We study this model in three regimes: at finite temperatures, assuming that the gauge field is purely transverse; at T=0, for the purely static (2D Coulomb) interaction; and at T=0, for a weak Coulomb interaction and a strong transverse one. Transverse interactions suppress the temperature of the superfluid transition significantly. A sufficiently strong transvese interaction is shown to produce a phase separation as the temperature decreases (in the absence of Coulomb repulsion). If there is Coulomb repulsion, the ground state does not have off-diagonal long-range order but the superfluid density is not zero unless the Coulomb constant exceeds a critical value. Sufficiently strong coupling to the transverse field destroys superfluidity as well. In the normal state formed at large couplings, the translational invariance is intact. We propose a bosonic ground state that is not superfluid at T=0. We discuss the implications of these results both for vortex liquids and strongly correlated electron systems.

The one-dimensional Hubbard model with repulsive interaction is studied. We calculate the asymptotic correlation functions in the less-than-half-filled band case. In particular, our approach centers on the large-U wave function and emphasizes the finite phase shifts on the Fermi surface. This is important since the concept of finite phase shifts can be easily applied to higher dimensions. We then discuss the non-Fermi-liquid behavior of this model and the relevance to the understanding of the two-dimensional Hubbard model, which is believed to give a good description of the high-${\mathit{T}}_{\mathit{c}}$ superconductors.

The variation of the superconducting order parameter (extended s wave) with hole concentration is studied for a narrow-band system, described by an extended Hubbard model, using the renormalized Hamiltonian approach (RHA) and X-operator technique in the strong-correlation limit and by the Hartree-Fock approximation for the weak correlation case. A comparison of the results obtained by using the RHA and the X-operator technique are made for different densities of states. Analytical results for the superconducting order parameter and its relation to the delocalization energies in the normal and superconducting states are obtained at zero chemical potential.

The thermoelectric power (TEP) of textured ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ has been measured with an ac technique in an applied magnetic field (up to 3 kG). Parallel and perpendicular geometries of the temperature gradient and the magnetic field have been systematically studied. A field-induced hysteretic behavior of the TEP due to the difference between cooling and heating runs is observed. The hysteresis surface versus field curve presents a maximum near 100 G for both geometries. The hysteretic behavior below ${\mathit{T}}_{\mathit{c}}$ is discussed in terms of the intrinsic Josephson effect caused by the formation of Josephson vortices, while above ${\mathit{T}}_{\mathit{c}}$ the hysteresis is attributed to the manifestation of the two-dimensional Gaussian fluctuations of the order parameter. In so doing, the critical-current density is found to behave as ${\mathit{J}}_{\mathit{c}}$∝[1+(H/${\mathit{H}}_{\mathit{J}}$${)}^2$${]}^{\mathrm{-}1}$. An activation energy, U(T,H), deduced from the Arrhenius plot of the TEP below ${\mathit{T}}_{\mathit{c}}$, shows a linear temperature dependence, U(T)≊1-T/${\mathit{T}}_{\mathit{c}}$, a power-law magnetic field dependence, U(H)∝${\mathit{H}}^{\mathrm{-}1/4}$, and has the absolute value of ca. 0.5 eV at T=0 and H=50 G. Such a behavior of the observed TEP and the dependences and origin of U(T,H) are argued to be attributed to the highly anisotropic nature of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ layered superconductors.

In this paper we have studied the role of bonding, reduced screening, and structure in high-temperature superconductors (HTSC) using ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ as a prototype material. In addition to the ionic and metallic component of bonding recently investigated, we estimate the influence of covalence effects on the structural parameters and the phonon spectrum. Also, the effect of a reduced dispersion of the electronic band structure on the polarizability and the phonon frequencies is considered and (hypothetical) mechanisms are investigated, which would favor ferroelectricity in the HTSC. In a further topic we deal with reduced screening, introduced by nonadiabatic effects related to the quasi-two-dimensional band structure and its consequence for the electron-phonon interaction, the phonon dispersion, and the pairing mechanism. We then focus our interest on features related to the tetragonal-to-orthorhombic phase transition in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. Structural parameters are calculated and the influence of the transition on the phonon dispersion as well as on the phonon-induced charge fluctuations is discussed. Finally, we present, on the basis of nonlocal electron-phonon interaction of the charge-fluctuation type, a theoretical interpretation of a universal relationship between ${\mathit{T}}_{\mathit{c}}$ and the hole content in the HTSC.

Temperature and spectral dependencies of photoinduced changes of resistance in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ thin films with oxygen content ranging as 6.35<x<6.75 were measured on both sides of the metal-insulator transition (MIT). The absolute value of efficiency of initiation of photoinduced changes decreases with an increase of oxygen content, but the position of peaks in the spectral dependence does not change with a change of x. Temperature dependencies of efficiency have an anomaly at T∼220 K, which is present in all the samples studied and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependencies of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in Y-Ba-Cu-O on both sides of the MIT has a common origin.

The semiconducting parent compounds of the high-${\mathit{T}}_{\mathit{c}}$ superconducting system ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$, at low oxygen content (0<x<0.25) and different oxygen isotopes, have been investigated by means of infrared (IR) excited (1.16 eV) Raman scattering and photoluminescence (PL). The IR-excited Raman spectrum of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ is dominated by a group of three bands in the spectral range of the apex oxygen O(4) ${\mathit{A}}_{\mathit{g}}$ mode (475 ${\mathrm{cm}}^{\mathrm{-}1}$ in the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ compound). The Raman spectrum changes dramatically upon oxygen doping with the appearance of a strong resonant band at 507 ${\mathrm{cm}}^{\mathrm{-}1}$ accompanied by several overtones. By studying site-selective ${}_{}{}^{18}{}_{}{}^{}\mathrm{substituted}$ samples this band is assigned to ${\mathit{A}}_{\mathit{g}}$ vibrations of apex O(4) atoms adjacent to the O(1) oxygen atoms occupying a chain site upon doping. The energy shift of this phonon and its associated overtones is caused by a strong lattice relaxation around the O(4)-Cu(1)-O(4) complex due to electronic charge-transfer processes involving O(4) sites. This behavior implies a strong coupling between O(4) vibration and charge-transfer excitations. PL, with a maximum at about 1.3 eV, has been observed in samples with very low oxygen concentrations. Peak energy and intensity are temperature and doping dependent. Both Raman and PL data are interpreted assuming the existence of a narrow band, associated with the O(4)-Cu(1)-O(4) complex (in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$) which is located, for small x, within the ${\mathrm{CuO}}_2$ plane charge-transfer gap.

The fluctuating properties of the surface superconducting layers created by an electric field perpendicular to the surface are investigated. Shifts of the critical temperature, heat capacity, and the conductivity above the critical temperature have been calculated for arbitrary relations between the screening and coherence lengths.

The effect of substitution at the Cu site by the transition elements Fe, Co, Ni, and Zn on the structure and superconducting properties of ${\mathrm{NdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ has been investigated. Compared to the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{3\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{O}}_{7+\mathrm{\delta }}$ (M=Fe, Co, Ni, Zn) system, considerably high solid solubility and drastic ${\mathit{T}}_{\mathit{c}}$ suppression is achieved in the Nd 1:2:3 system. The maximum solid solubilities achieved for the dopants are x=1.0 for M=Fe and Co and x=0.8 for M=Ni and Zn. An orthorhombic-to-tetragonal (O→T) transition is observed at x=0.08 for M=Fe, Co similar to the case of the M-doped Y 1:2:3 system. However, a second T→O transition is observed in Nd 1:2:3 at x=0.5 for M=Co and x=0.8 for M=Fe. For the Ni- and Zn-doped phases, an O→T transition is observed at x=0.4 for M=Ni and x=0.5 for M=Zn. The doped phases are superconducting up to x=0.2 (M=Fe, Co, Ni) and x=0.08 (M=Zn). The initial rate of ${\mathit{T}}_{\mathit{c}}$ suppression (up to 3.33 at. %) is ∼14 K/at. % for M=Fe, Co, and Ni and ∼28 K/at. % for M=Zn and is much higher than that reported for the Y 1:2:3 system. The Fe- and Zn-doped phases, heated in flowing ${\mathrm{N}}_2$ gas at 800 °C followed by reoxygenation at 450 °C show higher ${\mathit{T}}_{\mathit{c}}$ and sharper superconducting transition. Hole doping by Ca substitution (y) at the Nd site in the Zn-doped (x=0.1) semimetallic-semiconducting Nd 1:2:3 phase induces metallicity and superconductivity for y≥0.1 and a ${\mathit{T}}_{\mathit{c}}$ of 32 K is obtained for y=0.2.

Magnetoresistance measurements on superconducting ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ single crystals with ${\mathit{T}}_{\mathit{c}}$ varying from 7 to 19 K were made with magnetic fields H applied parallel (H∥c) and perpendicular (H⊥c) to the tetragonal c axis. For H∥c, the resistive superconductive transition curves gradually develop structure at low temperatures and high magnetic fields as ${\mathit{T}}_{\mathit{c}}$ decreases. For the crystals with ${\mathit{T}}_{\mathit{c}}$≲11 K, this structure evolves into a double resistive transition with a resistivity maximum near T≊1.1 K, followed by an extremely sharp transition into the superconducting state. In this region, the resistivity exhibits non-Ohmic behavior for low current densities and Ohmic behavior for larger measuring currents. The resistivity for H⊥c does not exhibit any anomalous behavior or a maximum. The irreversibility line inferred from these measurements is sample dependent.

We report Raman-scattering measurements on several superconducting superlattice films of (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$${)}_{\mathit{m}}$(${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$${)}_{\mathit{n}}$. The superstructure period varied between 4<(m+n)<12 and was oriented with the c axis of the initial bulk crystalline materials lying normal to the film surface. The observed Raman-active vibrations of the superlattice are readily identified through comparison with spectra from the bulk components. The dependence of the vibrational frequencies on m and n shows that phonons associated with the apical oxygen atoms (${\mathrm{O}}_4$) are mainly localized to the Y and Pr layers for the larger period structures and transform to an extended excitation as the modulation period is reduced. This behavior is consistent with charge redistribution across the interface and may account for the drop in the transition temperature observed in thin Y-layer superlattices. The degree of interdiffusion between Pr and Y sites is shown to be negligible with no detectable interfacial alloying. Reminiscent of behavior in bulk ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, the out-of-phase O(2)-O(3) phonon confined to Y layers as small as two unit cells softens below the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. The corresponding phonon localized to the Pr layers shows no softening down to n=1 and therefore suggests the absence of proximity-induced superconductivity in the ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layers.

A Landau free energy for the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$(Ba,Sr${)}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ family is constructed, including the coupling energy between the order parameter and strain tensor. Using group theory, it is demonstrated that the coupling energy can be expressed as a sum of terms linear in the strain and quadratic in the order parameter and should contain a coupling term bilinear in the two order-parameter components. The structural phase transitions (SPT’s) in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$(Ba,Sr${)}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ are discussed on this basis. It is shown that several SPT’s may occur in this system under different conditions. The observations of the influences of applied pressure, doping concentration x, and oxygen deficiency y on the SPT temperature are interpreted quantitatively. Relations connecting spontaneous strain, soft-mode vibrational frequencies, heat capacity, and elastic constants of the lower-temperature phases are derived. The mean-field results of the elastic properties and the results of the scaling analysis are also given. The results of ultrasonic measurements on a ${\mathrm{La}}_{1.88}$${\mathrm{Sr}}_{0.12}$${\mathrm{CuO}}_4$ single crystal are reported. The present and other relevant experimental observations are compared with the obtained theoretical results. The possible effects of structural phase transitions or structural instabilities on the superconductivity of the system are discussed.

Zero-field copper NMR measurements on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_6$ up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering results are pointed out and justified in terms of three-dimensional stacking defects.

We report a magnetic-resonance study of Gd-doped ${\mathrm{Eu}}_2$${\mathrm{CuO}}_4$ single crystals. Cooling the samples in a magnetic field ${\mathbf{H}}_{\mathrm{FC}}$, induces weak ferromagnetism (WF), with a strong out-of-plane anisotropy determined by the Dzyaloshinsky-Moriya (DM) interaction. In addition, there is in-plane anisotropy with an easy-axis parallel to the [110] crystal axis closest to ${\mathbf{H}}_{\mathrm{FC}}$. An intense resonance mode is observed at the X band (9.5 GHz) when ${\mathbf{H}}_{\mathrm{FC}}$ is applied parallel to one of the 〈110〉 axes and the measuring field is rotated by 90° in the ${\mathrm{CuO}}_2$ plane. At the Q band (35 GHz), the in-plane resonance modes strongly depend on angle and temperature. We analyze the experimental results in terms of a phenomenological model for the magnetic free energy, which predicts a reorientation transition of the WF component of the magnetization ${\mathbf{m}}_{\mathrm{WF}}$ induced by the external field. Associated with this transition, a softening of the WF magnetic resonance mode occurs when the external field is applied perpendicular to the easy magnetization axis. The resulting angular variation of the resonance modes depends on whether the energy gap for the magnetic excitations is larger or smaller than the microwave energy. From the resonance data we have determined both the out-of-plane and in-plane anisotropy fields, ${\mathit{H}}_{\mathrm{DM}}$(T) and ${\mathit{H}}_{\mathrm{ax}}$(T), respectively.

The extrapolated values for T=0 are ${\mathit{H}}_{\mathrm{DM}}$(0)=3.5(5)×${10}^5$ G and ${\mathit{H}}_{\mathrm{ax}}$(0)=12(2) G. Both anisotropy fields decrease with increasing T, vanishing around ${\mathit{T}}_{\mathit{N}}$≃243 K. The temperature dependence of the peak-to-peak linewidths, Δ${\mathit{H}}_{\mathrm{pp}}$, measured at the X and Q bands is explained in terms of a temperature-independent frequency linewidth, Δ${\mathrm{\omega }}_{1/2}$/γ=1.6(2) kG. Nonresonant absorption losses around the maxima and minima of the ω/γ vs H curves are also described in terms of this finite width for the resonance modes.

To interpret the linear frequency dependence of the Raman light-vibration coupling coefficient C(ω), the disorder in glasses is characterized by two different correlation lengths, radial (along a line) and angular (between lines).